When molecular ions generated from sample molecules are moved through a gas (or liquid) medium due to the action of an electric field, those ions move at a velocity proportional to their mobility, which is determined by the strength of the electric field, the size of the molecules, etc. Ion mobility spectrometry (IMS) is a measurement method which employs this mobility for analysis of sample molecules. FIG. 5 is a schematic of a common ion mobility spectrometer disclosed in Patent Literatures 1 and 2 and the like.
An ion mobility spectrometer comprises, inside, for example, an unillustrated round tubular housing, an ionization region 10 for ionizing component molecules in a sample, a drift region 11 for measuring the movement velocity of the ions, and a detector 14 which detects ions flying through the drift region 11. Furthermore, a shutter gate grid 12 is provided at the boundary between the ionization region 10 and the drift region 11 for feeding the ions generated in the ionization region 10 into the drift region 11 in the form of pulses limited to a very short time width.
The shutter gate grid 12 comprises a pair of grid electrodes 12a, 12b provided spaced apart at a predetermined interval d in the ion passage direction. FIG. 6 (a) is a schematic of the grid electrodes 12a, 12b viewed from the ion introduction direction. In this example, the grid electrodes 12a, 12b consist of multiple conductive wires 121, 122 which are stretched in parallel in the same plane. Conductive wires 122 in the rear grid electrode 12b are provided right in middle of two adjacent conductive wires 121 of the front grid electrode 12a, so when a pair of grid electrodes 12a, 12b is viewed from the ion introduction direction, as shown in FIG. 6 (b), the conductive wires 122 are lined up substantially at equal intervals, and the space between adjacent conductive wires 122 constitutes a slot-shaped opening through which ions can pass (for example, see FIG. 7 of Patent Literature 2).
In the above ion mobility spectrometer, an electric field (an electric field which accelerates ions) which exhibits a downward potential gradient in the ion travel direction is formed in the drift region 11 due to the direct current voltage applied to the annular electrodes 13 arranged in the ionization region 10 and drift region 11. When sample components in the sample gas are introduced into the ionization region 10, they are ionized in the ionization region 10 by the action of β rays emitted from a β ray source (63Ni, etc.) or the like. When ions generated in this manner pass through the shutter gate grid 12 and are introduced into the drift region 11 only in periods of very short time width, the ions proceed along a downward potential gradient. Furthermore, while not illustrated, diffusion gas flow in a direction opposite to the ion travel direction is formed in the drift region 11, and the ions move while colliding with this diffusion gas. In the process of this movement, the ions are separated according to their size, etc., and ions of different size arrive at the detector 14 with a time difference. The resolution of ion mobility spectrometry depends to a great extent on the time width (pulse width) of ions passing through the shutter gate grid 12, so in order to increase the resolution, it is necessary to make the pulse width of ions at the shutter gate grid 12 as short as possible.
FIG. 7 is a schematic of the potential gradient in the ion optical axis direction and the voltage applied to the pair of grid electrodes 12a, 12b. In this example, ions of positive polarity are assumed. When an identical voltage (Vref) is applied to the front grid electrode 12a and the rear grid electrode 12b, the shutter gate grid 12 assumes a substantially open state (see FIG. 7 (b)), and the ions pass through the shutter gate grid 12 and flow into the drift region 11. On the other hand, when voltage is applied to the grid electrodes 12a, 12b so as to make the potential of the rear grid electrode 12b several hundred V higher than the potential of the front grid electrode 12a, the shutter gate grid 12 assumes a substantially closed state (see FIG. 7 (c)) as a result of this potential barrier, and the ions are blocked by the shutter gate grid 12. Generally, the open time of the shutter gate grid 12 is several hundred μsec to several msec, and the opening frequency is about several tens of msec.
In order to narrow the pulse width of ions in a shutter gate grid 12 consisting of a pair of grid electrodes 12a, 12b as described above, it is necessary to make the separation distance d between the grid electrodes 12a, 12b as small as possible while avoiding contact between the conductive wires of the different grid electrodes. Furthermore, in order to obtain a high ion throughput efficiency, it is necessary to use conductive wires 121, 122 which are as thin as possible and to arrange multiple such wires in parallel at a narrow gap. To do this, high mechanical precision is required for the arrangement of the grid electrodes 12a, 12b and the conductive wires 121, 122 in each grid electrode. However, when the shutter gate grid is in a closed state, as described above, quite a large voltage difference is applied to the pair of grid electrodes 12a, 12b, so the force produced by the electric field acts upon the conductive wires, and the conductive wires of the front grid electrode 12a and the conductive wires of the rear grid electrode 12b are attracted toward each other. For this reason, it is difficult to precisely maintain the separation distance d of the grid electrodes 12a, 12b. 
A technique for arranging multiple conductive wires in rows in a plane with high precision while avoiding contact between conductive wires of different grid electrodes and making the separation distance d as small as possible is disclosed in Patent Literature 3. Here, holes are formed at suitable locations in a flange extending from the inner circumferential side of an annular member composed of an insulator, and two long conductive wires are passed through different holes, thereby forming rows of parallel conductive wires in one plane, corresponding to one grid electrode, and forming rows of parallel conductive wires on another plane, corresponding to the other grid electrode.
However, a shutter gate grid of this sort has a complex structure, and the assembly operation is time-consuming. Thus, it has the problem of increased cost.
Furthermore, since an insulative material is located very close to the ion flow passing through the shutter gate grid, there is the problem that charge-up occurs readily due to the ions contacting the insulative material.
Moreover, since the flange extending from the inner circumferential side of the annular member narrows the channel for the diffusion gas which flows opposite to the ion flow, there is the problem that the flow velocity of the diffusion gas becomes too great and resolution is reduced.
In addition, since the diffusion gas flows only in the central portion of the housing inside which the drift region is formed, charge-up and contamination tend to remain near the walls of the housing, so much time is expended on the initial conditioning, and there is the concern that such contamination may affect analytical sensitivity and resolution.